Garmin is a favorite for GPS charting. Easy to use, good mapping chips/programs available for them.
Lowrance is also popular. I inherited a head unit with the boat, then added the transponder, gps antenna, and a chart chip. The 500/5000 series are the 5" screen, available as stand along or combo units, color and mono, and are reasonable in cost. You can add the accessories as you require/can afford. Consider spending a little more and get a combo plotter/finder. They provide a lot of information at a reasonable cost and are easy to work with.
When I got my unit hooked up it didn't work. I contacted Lowrance and without any proof of purchase, receipts, nothing, they had me send the head unit in and they replaced it, and quick. That's customer service, and I've heard the same thing from others.
I also have a handheld Garmin unit, a 60cs, with the BlueChart maps. It works remarkably well, however as mentioned, the small screen is hard to use for navigating. When you're bouncing in a seaway, forget about being able to view any chart detail on a small screen.
I use both when navigating a distance. I put a way point up on the handheld unit with the navigating rose, and the charting up on the Lowranced unit. I get all the detail I need, and can switch the combo unit over for fish finding and sounding. They are nice companions to each other. The finder also reports water temp, nice to have for fishing temp breaks.
Many of the more portable, self-contained chart units have the GPS antenna built in. The Lowrance combo units have nothing, it's just the display, so you have to get the accessory sending units to get it to do all of the available functions. This can add substantially to the cost.
There are some good used bargains as well. Depending on the model and the cost, could be worth considering. I picked up my GPS antenna and chart chips online for less than half of retail.
There are other brands and all are reported to be good quality. My experience is with the cheaper, simpler, units. The serious offshore guys all go Furuno. But they have mega bucks into their boats too, and room for the bigger displays, need for radar, and etc.